Article: The origin of articulate crinoids
Cladistic analysis of various Palaeozoic and post-Palaeozoic crinoids indicates that the latter constitute a monophyletic clade, the Articulata, whose origins lie among the late Palaeozoic Ampelocrinidae of the inadunate order Cladida. The Cladida, raised to subclass alongside the Disparida and Camerata, is extended to include the Flexibilia and Articulata. Early articulates differ from some Palaeozoic cladids only in the absence of an anal plate in the adult cup, but a suite of characters can be used to identify progressively more derived members of the articulate stem group. Re-evaluation of two stem-group articulate families, the Ampelocrinidae and Cymbiocrinidae, indicates that there is little to justify retaining them as distinct families and that fewer than half of the constituent genera should be retained there. The remainder have been either wrongly assigned (Allosocrinus, Halogetocrinus and Paracymbiocrinus) or are based on material inadequate for establishing phylogenetic position (Armenocrinus, Arroyocrinus, Moundocrinus, Oklahomacrinus, Polusocrin Spheniscocrinus, Aenigmocrinus and Lecobasicrinus). Furthermore, several genera (including Nowracrinus and Tribrachycrinus) currently excluded show clear affinities with the Ampelocrinidae.